The Visual Viewport API provides an explicit mechanism for querying and modifying the properties of the visual viewport. The visual viewport is the visual portion of a screen excluding on-screen keyboards, areas outside of a pinch-zoom area, or any other on-screen artifact that doesn't scale with the dimensions of a page.

Visual Viewport concepts and usage

The mobile web contains two viewports, the layout viewport and the visual viewport. The layout viewport covers all the elements on a page and the visual viewport is what is actually visible on the screen. When the user pinch-zooms into the page, the visual viewport shrinks but the layout viewport is unchanged. User-interface features like the on-screen keyboard (OSK) can shrink the visual viewport without affecting the layout viewport.

What happens when a web page element needs to be visible on screen regardless of the visible portion of a web page? For example, what if you need a set of image controls to remain on screen regardless of the pinch zoom level of the device? Current browsers vary in how they handle this. Visual viewport lets web developers solve this by positioning elements relative to what's shown on screen.

The visual viewport API adds two events, onresize and onscroll, that fire whenever the visual viewport changes. These events allow you to position elements relative to the visual viewport that would normally be anchored to the layout viewport.

Example

The code below is based on the sample the specification, though it adds a few things that make it function better. It shows a function called viewportHandler(). When called it queries the offsetLeft and height properties for values it uses in a CSS translate() method. You invoke this function by passing it to both event calls.

One thing that may not be clear in this example is the use of the pendingUpdate flag and the call to requestAnimationFrame(). The pendingUpdate flag serves to prevent multiple invocations of the transfrom that can occur when onresize and onscroll fire at the same time. Using requestAnimationFrame() ensures that the transform ocurrs before the next render.